The large majority of electric generating stations in the United States rely on steam production to generate electricity. Water is pressurized and boiled to produce superheated steam which expands its energy in mechanical work by driving a turbine; a dynamo connected to the turbine simultaneously produces electric power. It is crucial to the economic operation of the plant that the boiler feedwater and steam be kept essentially free of impurities. Contaminants such as air and other gases can alter fluid pH and otherwise greatly increase corrosion potential. Moreover, the ingress of solid contaminants such as sulphates, chlorides, phosphates, nitrates and other dissolved or volatile salts will promote excessive corrosion and scaling of boiler tubes, turbine blades and other exposed components.
One conventional method for detecting contaminants in steam is to sample the steam as it flows through a pipe. A multi-port sampling nozzle is inserted through the pipe wall, preferably extending across the entire diameter of the pipe. A sample is extracted, cooled and condensed for subsequent analysis. This method is fraught with difficulties and therefore not widely used.
In many ways it would be preferable to extract a sample of the steam condensate, continuously, if practicable, from the plant's condenser. (Typically, steam leaving a generating station's turbines is cooled and condensed in a barometric well or vacuum chamber called a condenser or more simply, a "hotwell"). Sampling from the hotwell would allow for a more reliable indication of the level of contaminants which have passed through the plant's entire water-steam cycle and would especially reflect contaminant levels from the turbine (i.e. oils and metals) and the condenser's cooling water tubes (i.e. coolant leakage). Hotwell sampling would be especially useful where sea water is used as the condenser coolant since in leakage of this high salt medium could accelerate corrosion throughout the power plant unless, of course, the leak is promptly detected and corrected.
Heretofore, there has not been effective apparatus for extracting to atmospheric pressure either a continuous or occasional "grab sample" from a hotwell. Since the phase change from steam to water in the condenser creates a vacuum which thereby reduces back pressure on the turbine, the act of sampling from the hotwell must also overcome the vacuum condition while not introducing air into the condenser.
Some power plant operators have devised ad-hoc systems for sampling their hotwells. The systems with which we are familiar have involved a large circulating pump (i.e. 10 gallons per minute) constantly pumping condensate from the hotwell and back into the hotwell. A portion of the return flow is restricted and, when sufficient back pressure is built up, a sample may be bled off. A major problem with such systems has been that the pumps are cumbersome and require considerable attention. Typically, the large pumps depend upon the circulatory water itself for cooling the armature and if the pump prime should be lost, due to power load swings or plant shut down for example, the pump motor may quickly burn out.
Others have tried to sample hotwells by complicated valving systems to isolate a portion of the condensate, bring it to atmospheric pressure and drain a grab sample. These systems have required manual operation and have not been able to provide continuous, on-line sampling capacity. Thus, another problem in sampling hotwell condensate is that an accurate volume of sample must be delivered to the analyzing instruments on a continuing basis without pressure surges that would damage the testing components or cause eroneous readings. There exists a need for a simple, robust sampler which can accomplish this challenging task and also provide related monitoring and testing functions. We are not aware of any commercially available sampler capable of providing this hotwell sampling function.